The present invention relates to a process for producing a compound-based superconductor wire.
Superconductor wires made of intermetallic compounds such as Nb.sub.3 Sn and V.sub.3 Ga are characterized by a number of continuous fibers of intermetallic compound which are buried in the matrix phase. Wires having such a construction exhibit a high superconductivity but because of the intermetallic compound used, they are inherently brittle and permit an elongation of only less than 1.0% and are very vulnerable to mechanical tensile stress and bending stress. Therefore, the reliability of manufacturing such superconductor wires and winding them into a coil is low, and what is more, they cannot be cooled with liquid helium effectively.
In recent years, attempts have been made to produce a superconductor by using the "tunnel effect", also known as the proximity effect or filament effect, in which a number of very fine, discontinuous fibers of superconducting compounds are buried in the matrix phase very close to each other. Unfortunately, the superconducting characteristics of wire produced by this method are too low to satisfy practical requirements. A superconductor wire made of discontinuous fibers of a compound such as Nb.sub.3 Sn is produced typically by a process in which Cu and Nb are melted to form an ingot with spherical or acicular particles of Nb scattered within the Cu matrix and the ingot is drawn to the final dimensions and Sn is diffused into the Cu matrix from its surface or a process in which a Cu-based metal tube is filled with a mixture of Nb and Cu powders and the tube is drawn to the final dimensions and Sn is diffused into the Cu matrix from its surface to form a coating of Nb.sub.3 Sn on the Nb fibers. In the former method, if Cu is mixed with more than 25 vol % of Nb, it becomes difficult to melt and cast the mixture in a mold, and an ingot containing a sufficient percentage of Nb to provide improved superconducting characteristics cannot be formed. In addition, the casting is very difficult to draw. For these reasons, it has been practically impossible to make a wire having good superconducting characteristics using this method. In the latter method, the Nb powder in the Cu matrix does not form a sufficiently elongated fiber upon drawing and consequently it often breaks during the drawing step and thereby fails to provide a structure wherein a number of discontinuous Nb fibers elongated in the drawing direction are buried within the Cu matrix. Therefore, both processes have a low reliability and are capable of producing only a wire having poor superconducting characteristics.